(1) Field of the Invention
The present invention relates to an optical beam scanning apparatus in the field of the technique of recording images by scanning optical beams such as laser beams. More particularly, the present invention relates to an optical beam scanning apparatus and method of the type effective for a system where a broad scanning width and a small focussed beam diameter are required, such as an output equipment of a printing plate-making machine or a film printer for forming an original plate for printed substrates.
(2) Description of the Related Art
Optical beam scanning apparatuses of this type adopted at the present are roughly divided into the following three classes, and they have merits and demerits.
According to the most developed so-called drum scanner system, a photosensitive material is wound around a rotary drum, main scanning is performed by rotating the drum by a motor, and subsidiary scanning is effected by moving an optical source unit in a direction substantially perpendicular to the direction of main scanning by a subsidiary scanning mechanism, whereby an image is recorded.
According to this system, sufficiently good performances can be obtained with respect to the recording size and the image quality. However, since main scanning is accomplished by rotating a large drum having a photosensitive material attached thereto, the revolution speed is restricted by a large moment of inertia of the drum, and hence, the recording speed also is restricted.
In order to eliminate this disadvantage, there is often adopted in combination a so-called multiple beam recording system in which a plurality of optical fibers are arranged in parallel to construct an optical source unit, means for focussing optical beams going out from the end faces of the optical fibers to the photosensitive material is used, and thus, a plurality of main scannings are performed by one revolution by using a plurality of optical beams. However, even if such means is adopted, it is difficult to obtain a recording speed higher than recording speeds attainable in the two systems, described below, where optical beams per se are scanned, and no rapid improvement in the future can be expected.
The flat field scanning system will now be described.
According to this system, optical beams going out from a light source is moved in the direction of main scanning by a rotary polygonal mirror rotated by a motor and are focussed by a lens to effect main scanning of optical beams on a photosensitive material. The photosensitive material is moved in a direction substantially perpendicular to the direction of main scanning to effect subsidiary scanning. Thus, an image is recorded.
According to this system, main scanning is performed by moving optical beams per se by using a so-called rotary polygonal mirror or rotary pyramidal mirror. Therefore, the moment of inertia of the rotary member can be reduced, and hence, the revolution speed can be increased relatively easily. Furthermore, a plurality of main scannings corresponding to the number of planes of the mirror can be conducted by one revolution, and therefore, a high recording speed can be obtained relatively easily.
However, in the case where a broad scanning width is required as pointed out above, because of limitations in the design and production of lenses, it is difficult to obtain a small focussed beam diameter and it also is difficult to maintain a constant beam diameter and a constant beam shape through the entire scanning width. Moreover, unevenness of scanning line intervals, one defect of this system, is required to decrease as the beam diameter is reduced, and it is difficult to control this unevenness of scanning line intervals to the required level.
For these reasons, according to the flat field scanning system, a sufficient recording speed can be easily obtained, but it is difficult to simultaneously attain a satisfactory recording size and a satisfactory image quality.
Moreover in the flat field scanning system, the focal distance of the lens is generally equal to or larger than the scanning width because of the limitation of design of the lens, and the size of the scanning optical system becomes relatively large.
The other system of rotating optical beams per se is a system in which a stationary drum (hereinafter referred to as "cylinder") is used, and optical beams are rotated and scanned in the interior of the cylinder to a photosensitive material attached to the surface of the cylinder to effect recording of images (Japanese Unexamined Patent Publication No. 63-158580).
According to this system, optical beams from a light source are reflected by a light-reflecting element rotated in the interior of a cylinder by a motor, and by the focussing action of a lens arranged in an optical path, main scanning of the focussed optical beams is performed in the circumferential direction on the photosensitive material attached to the surface of the cylinder, while subsidiary scanning is performed in the axial direction by a subsidiary scanning mechanism, whereby an image is recorded.
For attaching the photosensitive material to the surface of the cylinder the photosensitive material can be retained on the inner surface of the cylinder, or the photosensitive material can be retained on the outer surface of a cylinder composed of a transparent material. The former method will be explained in the following description. This method will be called "internal drum scanning system". The light-reflecting element used in this system will be called "rotary reflecting element" in the following description.
According to this system, optical beams coming substantially in parallel to the revolution axis are reflected substantially in the perpendicular direction by the rotary reflecting element rotating with the central axis of the cylinder being as the revolution axis, whereby the photosensitive material attached to the inner surface of the cylinder is scanned and light-exposed.
The reflecting direction need not absolutely be the perpendicular direction, but a direction different from the rectangular direction can be adopted as the reflecting direction, so far as any bad influences are practically brought about by increase of the distance between the reflecting point and the point of light exposure of the photosensitive material or increase of the magnitude of the optical beams on the surface of the cylinder in the axial direction of the cylinder, caused by adopting the reflecting direction different from the perpendicular direction.
This system is characterized in that a relatively large scanning width is obtainable with a condenser lens having a short focal distance, as compared with the above-mentioned flat field scanning system. The fact that a lens having a short focal distance results in an advantage that a small focussed beam diameter required for high image quality recording can be easily obtained.
Furthermore, the problem of unevenness of scanning line intervals, which is serious in the flat field scanning system, can be moderated by the short focal distance of the lens.
The effective diameter of the condenser lens may be almost equal to the incident beam diameter, and a large lens as used in the flat field scanning mechanism need not be used so as to cope with a broad incident angle.
It is easy to maintain a constant beam diameter and a constant beam shape on the surface of the cylinder almost along the entire periphery thereof by increasing the mating precision between the revolution axis of the rotary reflecting element and the central axis of the cylinder according to need.
In short, according to the internal drum scanning system, the image quality can be improved more easily than in the flat field scanning system.
Even in view of limitations owing to mechanical arrangements of inlet and outlet openings of the photosensitive material, a holding mechanism for the rotary reflecting element, the subsidiary scanning mechanism and the like, it is sufficient if a scanning width corresponding to about 70 to about 80% of the entire circumferential length of the cylinder can be obtained and the focal distance of the condenser lens is slightly longer than the radius of the cylinder, and therefore, a scanning width about three times the focal distance of the lens can be easily obtained.
As the rotary reflecting element, a rectangular prism or pentagonal prism can be used instead of the mirror.
The lens can be rotated integrally with the rotary reflecting element in an optical path formed between the rotary reflecting element and the photosensitive material. In this case, the moment of inertia of the rotary members disadvantageously increases, but the focal distance of the lens can be made shorter than the radius of the cylinder and a smaller focussed beam diameter can be easily obtained. Namely, there can be attained an advantage that the diameter of beams incident in the lens, required for obtaining a constant focussed beam diameter, can be reduced.
In short, the internal drum scanning system is advantageous over the drum scanner system in that since the moment of inertia of the rotary members is small, the revolution speed can be easily increased and also the recording speed can be easily increased.
The internal drum scanning system is advantageous over the flat field scanning system in that the recording size can be easily made compatible with the image quality and the size of the apparatus can be easily reduced.
The internal drum scanning system has these merits, but this system is defective in that the recording speed cannot be increased so easily as in the flat field scanning system.
The reasons are as follows.
The first reason is that use of multiple beams (multi-beams) is difficult. The second reason is that it is difficult to perform a plurality of scannings of optical beams by one revolution using a rotary polygonal mirror or a rotary pyramidal mirror.
The reason why use of multiple beams is difficult is that in the case where a plurality of beams are reflected by the rotary reflecting element and scanned on the inner surface of the cylinder, the beams cross one another at two points on the inner surface of the cylinder and parallel dot rows necessary for recording of images are not formed.
In case of the flat field scanning system, a plurality of scannings can be performed by one revolution by using a known rotary polygonal mirror. However, in case of the internal drum scanning system, since the advantage of this system that a large scanning width can be obtained by a relatively small structure by performing scanning over an angle as broad as possible within the entire circumferential angle of 360 degrees should be effectively utilized, it is difficult to effect a plurality of scannings by one revolution by using a so-called rotary polygonal mirror or rotary pyramidal mirror.
When a rotary polygonal mirror or rotary pyramidal mirror is used in the internal drum scanning, the center of revolution is located a little bit distant from the central axis of the cylinder or the optical beams, and from the geometrical consideration, it is understood that the reflecting point is displaced in the plane rectangular to the central axis of the cylinder in the former case or toward the direction of the central axis of the cylinder in the latter case. Also for this reason, a rotary polygonal mirror or rotary pyramidal mirror can hardly be used.
Accordingly, in order to increase the recording speed by a single beam, it is necessary to rotate the rotary reflecting element at a very high speed.
For the foregoing reasons, in the internal drum scanning system, it is very difficult to attain a recording speed as high as attainable in the flat field scanning system.
As the means for solving this problem, there can be mentioned a method, as disclosed in Japanese Unexamined Patent Publication No. 59-119960 or Japanese Unexamined Patent Publication No. 57-151933, in which a plurality of light sources, optionally together with a driving current source, are mounted on a rotary member arranged within a cylinder and the light sources are rotated with the central axis of the cylinder being as the center to realize scanning of multiple beams.
According to this method, however, it is necessary to mount a plurality of light sources, electronic circuits for modulating the light sources, a generator for supplying electricity to the electronic circuits, power source-stabilizing circuits, and the like on the rotary member, and the moment of inertia of these members including a mechanism for holding these members stably on the rotary member becomes large, and therefore, the attainable revolution speed is limited.